Generally, diesel engines are internal combustion engines that draw air into a cylinder, compress the air to increase the temperature and pressure thereof, and inject a liquid fuel to the high temperature and high pressure air, thus causing spontaneous combustion and operating a piston, therefore obtaining power. The diesel engines may be classified into a direct injection type, a pre-combustion chamber type, a swirl chamber type, and an air chamber type according to the fuel inflow method. Among them, the direct injection type directly injects fuel into a combustion chamber under high pressure, and a fuel injection device of this type mainly includes a fuel injection pump, a fuel valve (injector), and a connecting pipe. Further, a unit injector that is constructed so that a fuel injection pump is coupled directly with an injector is also used as the fuel injection device.
A fuel injection pump is a machine that compresses fuel to high pressure and then transmits the fuel to the injector. In order to improve combustion performance and reduce exhaust gas, there has been a recent trend to increase the pressure of the injected fuel. This causes cavitation erosion in the barrel port of the barrel and the plunger constituting the fuel injection pump, thus bringing about a serious problem. That is, cavitation occurs even when fuel is injected under relatively low pressure, but the intensity of the cavitation is weak, so the degree of damage is not serious. Further, the damage occurs partially. Thus, by improving the design and changing the material of a damaged part according to the damage shape, it is possible to easily establish damage prevention measures. However, as the fuel injection pressure becomes high, the intensity of the cavitation also increases, so that cavitation damage occurs compositely in the barrel port of the barrel and the plunger, and the degree of damage also becomes very serious. However, in the prior art, the cause of cavitation damage has not been clearly investigated, so there are attempts to prevent cavitation damage using changes in design or materials based on existing experience.
For example, Korean Patent Laid-Open Publication No. 2001-0020139 discloses a fuel injection pump, in which an orifice member is installed in each change hole formed in a wall of a barrel to form considerably increased pressure in a space between the orifice member and a plunger, thus preventing cavitation from occurring in an area adjacent to the upper edge of the plunger. Further, Japanese Patent Laid-Open Publication No. Hei. 7-269442 postulates that damage to a plunger is caused by interrelation between a jet and the shape of a fuel outflow hole, and discloses a cavitation preventing apparatus for a fuel injection pump, which prevents damage to a plunger by forming a small hole for collapsing a cavity adjacent to the fuel outflow hole of a barrel. Japanese Patent Laid-Open Publication No. Hei. 7-54735 postulates that a cavity occurs and remains right before a barrel port is closed during a fuel intake process, and thereafter shock waves generated by a collision between fuel discharged from the barrel port and a deflector collide with the remaining cavity, thus leading to cavitation damage, and discloses a spill deflector for an internal combustion engine which is constructed so that a receiving hole opened or closed depending on the pressure of the discharged fuel is formed in an end of the deflector and fuel introduced through the receiving hole is dispersed to the outside of a barrel. Further, Japanese Patent Laid-Open Publication No. Hei. 5-340322 does not clarify the cause of cavitation damage, but asserts that the damage is caused by air bubbles remaining in a barrel port. This publication discloses a fuel injection device for an internal combustion engine, which is constructed so that a protective member having a fuel flow hole shaped to prevent the air bubbles from staying is provided on the outer portion of the barrel port, and fuel discharged from the barrel port when fuel injection is completed collides obliquely with the inner surface of the fuel flow hole of the protective member.
As such, in order to solve the cavitation damage that occurs compositely in the barrel port of the barrel and the plunger as the fuel injection pressure becomes high, various design improving methods have been proposed. However, these methods rely mainly on experience of damage shape without clearly revealing the cause of damage, so fundamental measures are not suggested.